1. Field of the Invention
The present invention relates generally to an automotive traction control system for suppressing wheel spinning or wheel slippage at driving wheels.
2. Description of the Background Art
Japanese Patent First (unexamined) Publication (Tokkai) Showa 61-85248 discloses one of typical construction of the prior proposed traction control system for the automotive vehicle. In the disclosed system, an engine output torque is lowered in response to occurrence of wheel slippage and a braking force is applied to the driving wheels for recovery of traction. In the shown system, slip control is initiated in response to average wheel slippage of left and right driving wheels. Such a way of control cannot be always satisfactorily precise because of a possibility of difference of road friction levels at left and right wheels.
Namely, in case wheel traction is to be recovered by applying a braking force for the driving wheels, the braking force tends to become insufficient for the driving wheel on a low friction road and unsatisfactory in suppression of wheel slippage. On the other hand, the braking force applied for the driving wheel on high friction roads tends to become excessive, and it unnecessarily lowers the wheel speed. Where wheel traction is recovered by lowering engine torque, excessive lowering of the engine output due to wheel slippage on the low friction road can cause degradation of engine acceleration characteristics and thus cause degradation of drivability of the vehicle.
Another prior proposed traction control system has been disclosed in the co-pending U.S. patent application Ser. No. 918,125, filed on Oct. 14, 1986 and assigned to the common assignee to the present invention. The same invention has also been disclosed in the German Patent First Publication 36 34 627. In the disclosure, a method and a system derive a value representative of an actual vehicle speed in strict correspondence to actual vehicle speed. The value can be utilized to detect wheel slippage and allows precise traction control, anti-skid brake control and so forth. The method for deriving wheel slippage, according to the present invention, includes a step of deriving a projected vehicle speed, the rate of change of which varies with road surface friction. Wheel slippage is derived from on the projected vehicle speed and instantaneous wheel speed. In practice, the method includes a step for detecting the force exerted on a vehicular wheel, which force includes a resistance variable depending upon the road surface friction .mu.. Based on the detected force, the projected vehicle speed variation pattern is derived so as to derive an instantaneous projected vehicle speed.
A further prior proposed traction control system has been disclosed in the co-pending U.S. patent application Ser. No. 918,137, filed on Oct. 14, 1986, and assigned to the common assignee to the present invention. This co-pending U.S. patent application discloses a traction control system which has a plurality of wheel speed sensor means for producing wheel speed indicative signals respectively representative of wheel speeds of associated vehicular wheels. Wheel slippage is detected by comparing wheel speed measured by the wheel speed sensor means and an approximated vehicle speed indicative value. The approximated vehicle speed indicative value is derived based on one of the wheel speeds selected depending upon a vehicle speed.
These traction control systems owned by the owner of the present invention are successful in some aspects; however, there space for improvement in order to obtain better traction control performance.